Edit in JSFiddle

.center{
    display: table;    /* table */
    width: 200px;
    height: 200px;
    color: #fff;
    background-color: black;
    
    position:absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    
    margin: auto;
}

.p {
    display:table-cell;    /* table-cell */
    text-align:center; 
    vertical-align:middle;    /* table-cell 에서 적용*/
}

<div class='center'>
    <p class="p">Center</p>
</div>